Amazon KDP offers authors an option to make their title exclusive to Amazon for a period of 90 days (meaning the title should not be available or appear to be available for sale for the selected 90 days period) and in return gives 5 promotional days where the eBook can be sold for free.
This program is called KDP Select Program and it also offers authors with 70% royalty for sales made during this period (0 royalty for free eBook sales).
If you are not satisfied with this option and are looking to sell your eBook for free for a longer period of time, then you should sell your eBook on KOBO. Usually, an author sells their work for free to acquire more readers and you should note that KOBO recently has been concentrating on Indian readers and is very friendly for new reader acquisition.
Example: If you promote your free eBook on Facebook and direct them to KDP, the reader will have to be an Amazon customer to continue buying or reading the book. Whereas with KOBO, the reader has an option to sign-in with Facebook/Google and continue reading instantly.
Hence promoting your eBook for a longer time and new reader acquisition and sign-up experience is better with KOBO than Amazon KDP for a reader who’s new to both of these platforms.